1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrophotographic copier. In particular, the invention relates to an improvement of an image forming apparatus of a type in which a visible image such as a full-color image formed on an image carrying body is transferred collectively to a transfer member.
2. Description of the Related Art
For example, an intermediate transfer type color image forming apparatus is known which is equipped with a latent image carrying body such as a photoreceptor drum on which a toner image of each color component corresponding to an electrostatic latent image is formed according to, for example, the electrophotographic scheme, a belt-like or drum-like intermediate transfer body to which each toner image on the latent image carrying body is transferred intermediately, a first transfer device for sequentially transferring toner images on the latent image carrying body to the intermediate transfer body, and a second transfer device (collective transfer device) for performing collective second transfer, on a transfer member such as a sheet, of the toner images that have been transferred to the intermediate transfer body (refer to Japanese Patent Application Laid-Open No. 5-323704 (1993), for example).
For example, a transfer roller is used as the second transfer device. The transfer roller gives a transfer member charges of the opposite polarity to the toner charging polarity, thereby transferring toner images formed on the intermediate transfer body to the transfer member by electrostatic force.
One example technique for peeling a transfer member from the intermediate transfer body after toner images of the respective color components on the intermediate transfer body have been transferred to the transfer member is a method of forcibly peeling the transfer member by bringing a peeling nail into sliding contact with the intermediate transfer body. Another method is available in which a peeling charge elimination device (e.g., a separation corotron) is provided downstream of the second transfer position and a transfer member is peeled by weakening the electrostatic adsorption force of the transfer member by eliminating charges from the transfer member through an AC corona discharge that is caused by the peeling charge elimination device (refer to Japanese Patent Application Laid-Open No. 8-146707 (1996), for example).
However, the technique in which the peeling nail, for example, is used to peel a transfer member from the intermediate transfer body has a technical problem that a transfer member and the intermediate transfer body are prone to formation of a scratch.
On the other hand, the technique using the peeling charge elimination device has a problem that when charges are eliminated from a transfer member after a second transfer, the amount of charge on the transfer member decreases and the electrostatic adsorption force between toner images on the transfer member and the transfer member is lost, causing toner scattering.
To solve the above problems, a method has already been proposed in which a conductive peeling plate is provided, for example, downstream of the second transfer position. A transfer member is peeled by utilizing a phenomenon that when the transfer member approaches the peeling plate after passing the second transfer position, charges that are reverse in polarity to charges that have been given to the transfer member at the second transfer position are induced on the peeling plate and hence the transfer member is electrostatically adsorbed on the peeling plate (refer to Japanese Patent Publication No. 8-23719 (1996), for example).
However, even in this type of apparatus, if the volume resistivity of the intermediate transfer body becomes, for example, 12 Log xcexa9xc2x7cm or larger because of variations of electric characteristic values of the material of the intermediate transfer body due to environmental variations (i.e., variations in temperature and humidity), after a second transfer a transfer member is given negative charges through a peeling discharge between the transfer member and the intermediate transfer body and positive charges that have been induced by electrostatic polarization remain on the surface of the intermediate transfer body.
At this time, if the volume resistivity of the intermediate transfer body is high, the potential of the surface of the intermediate transfer body becomes, for example, +2,000 V or more and, in addition, the increased volume resistivity markedly elongates the time that is taken until the charges on the intermediate transfer body decrease sufficiently. This results in fear that the electrostatic adsorption force acting on the transfer member that is caused by charges on the intermediate transfer body becomes stronger than the electrostatic adsorption force cased by charges induced on the conductive peeling plate that is provided close to and downstream of the second transfer portion, whereby the transfer member is kept stuck to the intermediate transfer body, that is, a peeling failure occurs.
As described above, after a second transfer the intermediate transfer body continues to hold an electric field in such a direction as to attract a transfer member.
At this time, if a transfer member jams at a location other than the second transfer position, there is a possibility, depending on the timing, that a transfer member is located in a region where a portion of the intermediate transfer body that has been subjected to a second transfer is charged.
In such circumstances, there is fear that the transfer member that has once been peeled from the intermediate transfer body again sticks to the charged portion of the intermediate transfer body.
Conventional image forming apparatuses have no means for preventing such re-sticking of a transfer member; once a transfer member is re-stuck to the intermediate transfer body, it is difficult for a user to realize that fact.
Therefore, there is fear that if such re-sticking of a transfer member occurs, a user does not realize it and the transfer member is transported being stuck to the intermediate transfer body, resulting in a jam at a location that the user is hard to reach to remove the jammed transfer member such as the position of an intermediate transfer body cleaner.
In particular, the possibility of occurrence of the phenomenon of re-sticking of a transfer member to the intermediate transfer body is higher in the case of a transfer member that is easily bent or a transfer member having an extremely large resistivity value (e.g., second-surface copying of a double-sided copying operation in a low-temperature, low-humidity environment).
Although as described above the above technical problems are remarkable in intermediate transfer type image forming apparatus, those problems also occurs in, for example, image forming apparatuses of a type in which a toner image on a photoreceptor belt is directly transferred to a transfer member.
The present invention has been made to solve the above problems in the art, and provides an image forming apparatus which effectively prevents a transfer member from re-sticking to an image carrying body at the occurrence of an abnormality such as a jam.
As shown in FIG. 1, the image forming apparatus has an image carrying body 1 for carrying a visible image formed by charged particles, a collective transfer unit 2 for collectively transferring the visible image on the image carrying body 1 to a transfer member 3, a stopping unit 4 for stopping devices during image formation, a transfer member presence judging unit 5 for judging whether the transfer member 3 exists downstream of and in the vicinity of a transfer position of the collective transfer unit 2, and a pre-stop processing unit 6 for preventing, if the transfer member presence judging unit 5 judges that the transfer member 3 exists when the stopping unit 4 stops the devices, the transfer member 3 from electrostatically sticking to the image carrying body 1.
The image forming scheme that is employed in the image forming apparatus according to the invention may be the electrophotographic scheme, the electrostatic recording scheme, or other proper schemes. Either a single or multiple image carrying bodies 1 may be used to form images of respective color components.
The image carrying body 1 may be belt-like or drum-like as long as it can carry a visible image. The form of the image carrying body 1 may be selected as desired from among various forms. For example, naturally, the image carrying body 1 may be formed by only an image formation carrying body 1a made of a photosensitive material or a dielectric. Alternatively, the image carrying body 1 may have such an image formation carrying body 1a and a belt-like or drum-like intermediate transfer body 1b which a visible image on the image formation carrying body 1a is transferred to and held on temporarily.
The collective transfer unit 2 may be of the transfer roller type, the corotron type, or other proper types as long as it has a function of transferring a visible image on the image carrying body 1 to the transfer member 3. However, from the viewpoint of easiness in controlling a transfer electric field, the transfer roller type is preferable in which case a transfer electric field can be controlled in a narrow range.
The stopping unit 4 maybe of any type as long as it forcibly stops devices at the occurrence of an abnormality such as a jam. The time points when the respective devices are stopped need not always be the same. Usually, a device where an abnormality such as a jam has occurred is stopped immediately and devices on which the pre-stop processing unit 6 act are stopped later than the other devices.
The transfer member presence judging unit 5 may be of any type as long as it judges whether the transfer member 3 exists downstream of and in the vicinity of the transfer position of the collective transfer unit 2. An affirmative judgment is made not only in a state that the rear edge of the transfer member 3 has passed the transfer position of the collective transfer unit 2 but also in a state that the transfer member 3 is passing the transfer position of the collective transfer unit 2.
The transfer member presence judging unit 5 may be selected as desired from among various types. For example, the transfer member presence judging unit 5 may be such that one or more position sensors for detecting the front edge or rear edge of the transfer member 3 are disposed in its transport path and the presence of the transfer member 3 is directly judged based on signals supplied from the respective position sensors. Alternatively, the presence of the transfer member 3 may be judged by recognizing the position of the transfer member 3 by using a timer and a signal that is supplied from a prescribed position sensor.
The pre-stop processing unit 6 may be of any type as long as it prevents, if the transfer member presence judging unit 5 judges that the transfer member 3 exists downstream of and in the vicinity of the transfer position of the collective transfer unit 2 when the stopping unit 4 stops devices, the transfer member 3 from electrostatically sticking to the image carrying body 1.
A specific example of the pre-stop processing unit 6 is an electric field attraction unit for attracting, by means of an electric field, the transfer member 3 in the direction that it goes away from the image carrying body 1.
In this case, for example, the electric field attraction unit as the pre-stop processing unit 6 applies a prescribed bias to a constituent member of the collective transfer unit 2 or a member that is disposed downstream of and in the vicinity of the transfer position of the collective transfer unit 2.
In the mode in which the electric field attraction unit as the pre-stop processing unit 6 applies the prescribed bias to the constituent member of the collective transfer unit 2, it is preferable to generate an electric field that acts in the direction that is reverse to the direction of a transfer electric field of the collective transfer unit 2.
In this mode, even if the transfer member 3 that has passed the transfer position of the collective transfer unit 2 is charged negatively, for example, a portion of the image carrying body 1 that has passed the transfer position of the collective transfer unit 2 can also be charged negatively, for example. Therefore, the rear end portion of the transfer member 3 and the image carrying body 1 repel each other in a region downstream of the transfer position of the collective transfer unit 2, whereby the transfer member 3 is prevented from sticking to the image carrying body 1.
In the mode in which the electric field attraction unit as the pre-stop processing unit 6 applies the prescribed bias to the member that is disposed downstream of and in the vicinity of the transfer position of the collective transfer unit 2, from the viewpoint of effectively preventing disorder in a transferred image on the transfer member 3, it is preferable that the prescribed bias has the same polarity as the charging polarity of the transferred image on the transfer image 3.
The member disposed downstream of and in the vicinity of the transfer position of the collective transfer unit 2 means a guide member (exit chute) for guiding the transfer member 3 to a transport member that is disposed upstream of a fusing unit or a peeling member such as a peeling plate for peeling the transfer member 3.
In this mode, when the bias having the same polarity as the charging polarity of a transferred image on the transfer member 3 is applied to the member disposed downstream of and in the vicinity of the transfer position of the collective transfer unit 2, charges having the polarity that is reverse to the charging polarity of the transferred image on the transfer member 3 are induced on the back surface of the transfer member 3 that is a dielectric and hence the transfer member 3 is attracted electrostatically by the member that is closer to the transfer member 3 than the image carrying body 1 is. Therefore, the transfer member 3 is prevented from sticking to the image carrying body 1.
Further, since charges of the polarity that is reverse to the charging polarity of the transferred image are induced on the back surface of the transfer member 3, the transferred image on the front surface of the transfer member 3 is held by the above-described induced charges and there is no fear that disorder occurs in the transferred image.
Another example of the pre-stop processing unit 6 is such that the image carrying body 1 is idled by a prescribed amount after the action of the transfer field of the collective transfer unit 2 is canceled.
In this example, even if the transfer member 3 that has passed the transfer position of the collective transfer unit 2 is charged negatively, for example, a portion of the image carrying body 1 that has passed the transfer position of the collective transfer unit 2 is rendered in a non-charged state. Therefore, electrostatic attractive force is hard to act between the rear end portion of the transfer member 3 and the image carrying body 1 in a region downstream of the transfer position of the collective transfer unit 2, and the transfer member 3 is made so much less prone to stick to the image carrying body 1.
Still another example of the pre-stop processing unit 6 is such that the transfer member 3 is transported toward the downstream side of the transfer position of the collective transfer unit 2 until the rear end portion of the transfer member 3 completely passes the transfer position of the collective transfer unit 2 and reaches a position having a prescribed distance from the transfer position.
In this example, the pre-stop processing unit 6 may stop the image carrying body 1 first at a time point when the rear end portion of the transfer member 3 has passed the transfer position of the collective transfer unit 2. However, from the viewpoint of simplification of control, it is preferable to stop the image carrying body 1 after completion of the operation of transporting the transfer member 3.
In this example, since the rear end portion of the transfer member 3 is located at a position that is sufficiently distant from a charged portion of the image carrying body 1, electrostatic attractive force is hard to act between the rear end portion of the transfer member 3 and the image carrying body 1 in a region downstream of the transfer position of the collective transfer unit 2 and the transfer member 3 is made so much less prone to stick to the image carrying body 1.
A further example of the pre-stop processing unit 6 is such that the image carrying body 1 is rotated in the reverse direction by a prescribed amount on condition that the rear end portion of the transfer member 3 has passed the transfer position of the collective transfer unit 2.
In this case, the rotation amount of the image carrying body 1 in the reverse direction may be set at a proper value that is in such a range that the transfer member 3 that is located downstream of the transfer position of the collective transfer unit 2 and a charged portion of the image carrying body 1 are made sufficiently distant from each other and do not influence each other.
In this example, even if the transfer member 3 that has passed the transfer position of the collective transfer unit 2 is charged negatively, for example, by virtue of the reverse rotation of the image carrying body 1 a charged portion of the image carrying body 1 is sufficiently separated from the transfer member 3. In this state, since the image carrying body 1 has no charged portion that is opposed to the rear end portion of the transfer member 3 in a region downstream of the transfer position, electrostatic attractive force is hard to act between the rear end portion of the transfer member 3 and the image carrying body 1 and hence the transfer member 3 is made so much less prone to stick to the image carrying body 1.
As for the timing of the start of operation of the pre-stop processing unit 6, the pre-stop processing unit 6 may always operate if the transfer member 3 exists downstream of and in the vicinity of the transfer position of the collective transfer unit 2. However, in view of the fact that the easiness of re-sticking of the transfer member 3 to the image carrying body 1 depends on the use conditions (the environment condition, the type of transfer member 3, the image formation mode, etc.) of the transfer member 3, from the viewpoint of performing an efficient control, the pre-stop processing unit 6 may start to operate when it is judged, in consideration of the use conditions of the transfer member 3, that a condition is established that the transfer member 3 tends to re-stick to the image carrying body 1.
Next, the operation of the above-described image forming apparatus will be described below.
Referring to FIG. 1, the stopping unit 4 stops devices during image formation. If the transfer member presence judging unit 5 judges that the transfer member 3 exists downstream of and in the vicinity of the transfer position of the collective transfer unit 2, the pre-stop processing unit 6 operates basically and prevents the transfer member 3 located downstream of and in the vicinity of the transfer position of the collective transfer unit 2 from electrostatically sticking to the image carrying body 1 when the stopping unit 4 stops the devices.